Core C1: The Clinical/Statistical Core serves to support the three research projects and the technology development project of the U19 research program. The research projects described elsewhere are focused on immunological responses to yellow fever virus (YFV) vaccination and dengue infection. Specifically, they will evaluate human immune memory, innate immunity, and immunological senescence to these viruses, while the technology development project will study effector and memory B cells and create novel monoclonal antibody reagents targeting these same pathogens. The clinical core has four specific aims: 1) To provide clinical study expertise and capacity to ensure the success of the U19 scientific agenda. The support activities to be provided include: study design; clinical protocol and informed consent form preparation; obtaining required regulatory approvals; outreach and education with communities in which recruiting will occur; recruitment, screening, and enrollment; clinical study conduct according to Good Clinical Practice (GCP) standards; volunteer safety monitoring; clinical specimen collection, processing, tracking, storage, and shipping; preparation of progress reports, final report, presentations, and publications. 2) To provide statistical and data management expertise that ensures the success of the U19 scientific agenda. The support activities to be provided include: consultation on study design, sample size calculation, protocol writing, data management, descriptive statistics for studies, data analysis, preparation of progress reports, final reports, presentations, and publications. 3) To perform clinical studies with YFV vaccine that will provide the clinical specimens and data necessary to accomplish the scientific aims of the research and technology development projects. 4) To perform a clinical study of natural dengue infection that will provide the clinical specimens and data necessary to accomplish the scientific aims of the research and technology development projects.